Exposure of proteins to ozone leads to rapid destruction of the aromatic amino acids. Because tryptophan residues are the most sensitive targets, detailed studies of the oxidation of free tryptophan and tripeptides composed of two alanine and one tryptophan residue were carried out in an effort to understand the mechanism of ozone attack. The results are consistent with the interpretation that the primary product of the ozone- tryptophan interaction undergoes spontaneous decomposition via different pathways to form N-formylkynurenine: kynurenine: and as yet unidentified product(s) in a constant molar ratio of 26:4.0:70.